I got this info from OS Support, hope it helps.

The runtime CLR used by OutSystems Applications is 2.0. Therefore it can only support .net framework from 2.0 to 3.5 running in OutSystems applicational code which includes extensions. If you need to call .NET 4.0 code you can create a 4.0 web application exposing web services which can then be consumed by Service Studio or Integration Studio. As that application can live in a different Application Pool than the OutSystems applications, that specific application can run CLR 4.0 code.

This will be addressed in the future, but there is no release date yet (as far as I know).